1. Field of the Invention
This invention relates to thermoplastic extrusion machines, and more particularly to a configuration of the bore and rotor arranged therein.
2. Prior Art
Physical and optical properties of low density polyethylene are improved by mechanical shearing of the molten material. Additives such as processing aids, stabilizers and coloring agents are usually put in the material during the mechanical shearing processing. Historically, this mechanical shearing has been done in the intensive mixers or in cold feed extruders by remelting material through the application of mechanical shear forces. Since this material is originally produced as a hot molten liquid, this remelting process involves intermediate pelletizing, cooling and drying steps in the originally produced material.
A variety of attempts have been made in the past to increase the capabilities of extruder machines or to increase the amount of work put into the thermoplastic material in the extruder machine. These attempts have included multi-flighted extrusion screws, or rotors, having primary and secondary channels associated therewith. The rotor would be divided into a plurality of sections, for homogenizing, feeding and pumping the extrudite therealong. One or more of the channels in one or more of the sections in the rotor, might have a dam to provide back pressure to the thermoplastic material being worked through the extruding machine between the outside diameter of the rotor and the inside diameter of the bore disposed about the rotor. A machine of this type is shown in U.S. Pat. Nos. 3,411,179 to Gregory et al, and 3,850,414 (now disclaimed) to Scharer.
The shearing action of these machines is improved by the back pressure created by the dams across the channels in the rotors. However, the back pressure affects the heat removal rate of the machine, and some types of thermoplastic material may require more or less back pressure, shear mixing and homogenizing, or heat removal rates than other types of thermoplastic material.
It is therefore an object of the present invention to provide a homogenizing extrusion machine with capabilities to extrude a variety of thermoplastic materials, including low density polyethylene by being able to vary or eliminate the homogenization or mixing therewithin.